Adaptive equalizers, in particular adaptive equalizers that use linear equalization, are utilized to compensate for linear distortion that occurs in communication paths and transmitting devices and receiving devices in the fields of wireless communications and optical communications. An adaptive equalizer is an equalizer that compensates for linear distortion by adjusting internal parameters according to errors included in a reception signal.
Adaptive equalizers are classified into the type that controls the tap coefficients of a filter of an equalizer using a known sequence referred to as a pilot signal or a reference signal and the type that controls tap coefficients without using a known sequence. Adaptive equalizers that control tap coefficients without using known sequences are also referred to as blind equalizers. A blind equalizer is utilized when no known sequence is included in a communication signal, for example.
The constant modulus algorithm (CMA) is known as a method for controlling the filter coefficient of a blind equalizer. The CMA has an advantage that the tap coefficients of an equalizer can be converged even when a signal for which the carrier wave frequencies of transmitting and receiving devices are not synchronized with each other is received. Patent Literature 1 discloses a method of following a propagation path fluctuation using the least mean square (LMS) algorithm with better follow-up performance than the CMA after obtaining tap coefficients using the CMA.
Generally, the clock frequencies of a transmitting device and a receiving device are not synchronized with each other. In other words, the clock frequencies of transmitting and receiving devices are asynchronous with each other. If the clock frequencies of transmitting and receiving devices are not synchronized with each other, when the receiving device samples a reception signal, a difference in symbol timing due to the clock frequency of the transmitting device occurs. Symbol timing is the timing at which a receiving device determines a symbol. Fractionally spaced equalization is used to correct the difference in symbol timing. An equalizer that realizes fractionally spaced equalization receives a signal sampled at a cycle shorter than a symbol interval, that is, an oversampled signal, and outputs a signal with the same rate as the symbol rate by a filtering process that uses the tap coefficients that are set by evaluating the error function of the CMA at the oversampling cycle. As a result, the equalizer that realizes fractionally spaced equalization can output a signal synchronized with symbol timing as well as subjected to channel distortion compensation. As described above, by using an equalizer that realizes fractionally spaced equalization, a signal synchronized with symbol timing can be generated without separately providing a function for symbol timing synchronization.